Carburizing is a widely used process for hardening the surface of steel parts by diffusion of carbon into the steel surface at high temperature. The carbon is supplied to the steel surfaces by a carrier gas having a high carbon potential. The most commonly used carburizing gas mixture is an endothermic gas which consists of 20% carbon monoxide (CO), 40% hydrogen (H2) and 40% nitrogen (N2). It is believed that the dominant reaction at the surface of the steel during the carburizing process is:CO+H2--->Csteel+H2O  (1)
The rate at which carbon is put into the steel is therefore proportional to the product of the CO and H2 concentration in the carrier gas. It is believed that the most productive reactive makeup for the reactant is a 50% CO and 50% H2 mixture. This mixture may decrease the carburizing times by as much as 50%, thereby doubling the furnace productivity. However, there is presently no practical and economical generator available to produce carburizing gas with high percentage of CO and H2. It would therefore be a benefit in the art to provide a process and apparatus for producing carburizing gas with a high percentage of CO and H2.
There have been various attempts in the art to increase the CO and H2 levels during the carburizing process. For example, one method includes direct injection of methanol into the furnace to generate an atmosphere with 33% CO and 67% H2. However, additional thermal load relating to methanol dissociation exists. Furthermore, this method is comparatively very expensive as the atmospheric costs associated with this method is calculated to be about 4.5 times the costs for the endothermic gas.
Another method involves direct injection of methane (CH4) and carbon dioxide (CO2) into the furnace, as in U.S. Pat. No. 5,676,769. Theoretically, this method can generate a 50% CO and 50% H2 mix in the furnace using the reaction:CH4+CO2--->2CO+2H2  (2)
This reaction is highly endothermic and requires a catalyst to proceed at the typical carburizing temperature of between about 900° C. to about 960° C. As a result, this makes the process difficult to control, requiring long residence times of the gas in the furnace. Long residence times have the disadvantage of generating H2O in the furnace while the steel picks up carbon, which in turn keep the carbon potential of the gas phase high. Another disadvantage is that the high methane concentration often leads to excessive soot buildup in the furnace. Therefore, a large amount of methane injection is required to reform the H2O and keep the carbon potential of the gas phase high. A major drawback to this method is that the high methane concentration often leads to excessive soot buildup in the furnace.
It is desirable to have a method that cures the deficiencies in the prior art. One such method is found in the present application having a high CO/H2 mixture that is prepared in a separate reactor by reforming a CH4/CO2 mixture over a noble metal catalyst. This avoids the problems associated with direct injection of CH4/CO2 into the furnace and is significantly less costly than the direct injection of methanol.